In recent years, it has attracted much attention to develop a technique for growing or regenerating a plant having a character or trait suitable for an intended purpose. Among these growing and regenerating techniques, a plant tissue culture technique, i.e., a technique wherein a part of a plant is separated from the main body thereof, and is grown in the inside a culturing vessel has particularly attracted much attention, since it enables the mass multiplication of a genetically identical clone in a short period of time (Kiyoshi Okawa "General Introduction to Horticultural of Flowers", page 54, 1995, Yokendo).
Heretofore, in the production of a plant (mainly, a plantlet) using a plant tissue-culturing technique, agar gel has been used as a support therefor. However, the agar gel has a characteristic such that it hardly absorb water content again, after it discharges water content due to the evaporation of water or the absorption of water into the plant. Particularly, in an open-system cultivation environment such as farm cultivation, the water content-retaining function of the agar and the plant-retaining function thereof as a gel are rapidly decreased. Accordingly, it is naturally impossible to use the agar gel as the support (or planting material) in the open-system farm cultivation.
From such a viewpoint, it is necessary to remove the agar gel at the time at which a plantlet is transferred from the culture thereof in a vessel into the farm cultivation thereof. However, it is inevitable to manually conduct a step of removing the agar gel one by one with respect to the plantlet, and therefore such a removing step not only requires much labor and long time, but also poses some problems such that it damages the root of the plantlet, is liable to cause a root decay (or root rot) phenomenon, etc.
On the other hand, in the case of the culture of a very small plantlet (minute plantlet), a saccharide is generally added into the liquid culture medium to be used for the culture. The reason for this is that, in general, such a minute plantlet does not have an organ such as albumen of a seed, leaf and stem portion thereof for conducting a photo-synthesis reaction is not sufficiently developed, and therefore it is necessary to add a carbon source which the minute plantlet can absorb directly. In addition to the case of the tissue-cultured plantlet, in a case of a seed having no albumen such as that of an orchid (Orchidaceae) plant, a minute plantlet which has been germinated in a vessel is subjected to a saccharide-involving (or saccharide-relating) culture process for a similar reason. However, since the saccharide which has been added into the plant-growing system promotes the propagation of various germs, it is substantially impossible to use the agar gel containing a saccharide as it is in an open-system (non-sterilized) environment such as farm cultivation.
As described above, in a case where the conventional method of growing a plant by using agar gel, it is substantially impossible to continuously grow the plant from the culture thereof to farm cultivation thereof (by using the support used for the culture as it is), and therefore the step of removing the agar is required. In addition, since such a agar-removing step must be conducted by using manual operations, a considerable period of time is required, and further, the step has problems such that it damages the root of the plant, and various germs are liable to propagate due to the presence of the remaining agar gel so as to cause a root decay phenomenon.
In addition to the above-mentioned problem of the transfer from the "culture" to the "cultivation", in the course of plant growth, it is also an important subject to control the amount of the supply of water content, a nutrient, etc., to the plant.
In view of the physiology of a plant, "water" is one of the environmental factors which exerts the greatest influence upon the growth of the plant, and particularly, it is an element essential for the photo-synthesis. The absorption of the water content, which is an extremely important environmental factor, is mainly concerned with the transpiration due to "stomata" as openings of the back surface of a leaf of the plant.
More specifically, when the water content of cells constituting a plant is decreased by the transpiration of water, the water content in the plant assumes a non-equilibrium state, and the plant absorbs water content in soil through the root thereof on the basis of the "transpiration pressure" as an action for retaining the non-equilibrium state in an equilibrium.
The above-mentioned stoma also has a function of taking CO.sub.2 in, which is necessary for the photo-synthesis, from the air. However, since the water content in the mesophyll cells, wherein the photo-synthesis is mainly to be conducted, is transpired due to the presence of the openings of the stomata, the water content in the mesophyll cells is required to be supplemented promptly. In other words, water must be supplied plentifully to the plant along with the absorption of solar energy and CO.sub.2 so that plant conducts the photosynthesis more effectively.
In the daytime environment wherein the temperature is high and the light quantity of the sunlight is large, when the water content which a plant can utilize is insufficient in the cultivating soil, or when the water-absorbing ability of the root of the plant is lowered, the water content in the plant is decreased, and the water content in the mesophyll cells wherein the photo-synthesis is mainly to be conducted is also decreased. As a result, not only the photo-synthesis is markedly inhibited, but also the photo-synthesis product is markedly decreased so that the growth of the plant per se is suppressed, and there is a danger such that the plant is withered to death in the course of time. Further, when the water content in soil becomes insufficient, the concentration of mineral salts contained in the soil is increased. On the contrary, when the water content in soil becomes excessive, the supply of oxygen to the root of the plant becomes insufficient. In both of these cases, there is a fear such that the plant is adversely affected.
On the other hand, "temperature" is also one of the environmental factors which greatly affects the growth of a plant. For example, the absorption of mineral salts through the root of a plant is increased along with an increase in temperature, but the absorption will have a maximum when the temperature reaches at a certain value, and is sharply decreased at a higher temperature than such a value. It is known that the maximum value of the mineral salt absorption is present in the neighborhood of 40.degree. C. in most of the plants. The absorption of nutrients in a lower temperature region is mainly based on a simple diffusion phenomenon, but the ratio of active nutrient absorption based on a biochemical absorption process is increased along with an increase in temperature. In a high-temperature region of 40.degree. C. or more, the deactivation of an enzyme system relating to the biochemical absorption process is regarded as a cause of sharp decrease in the nutrient-absorbing rate. Accordingly, the control of water in the cultivating soil and the control of the amount and concentration of nutrients with respect to a temperature change is very important technique for the cultivation of a plant.
On the other hand, from a viewpoint of technique of crop production, cultivation in open-air field has heretofore been conducted in a natural (or non-artificial) environment with respect to crops such as grain plant, vegetables, flowering plants, and fruit trees. However, in the open-air field cultivation, since the production amount of the crops are greatly varied due to a violent change in temperature during four seasons, unstable rainfall conditions, etc., the development of the agriculture as an industry has rather been limited.
In recent years, the cultivation of crops in facilities such as greenhouse has been popularized for the purpose of overcoming the above-mentioned problems in the open-air field cultivation or for the purpose of shipping the crops throughout one year (in any season constituting one year). As a result, it becomes possible to supply agricultural products stably.
However, in the facility cultivation, the production cost of the crops inevitably becomes high. The reason for this is that, in the facility cultivation, the facility-and-equipment investment becomes very large, which is necessary for the construction of the facility main body such greenhouse, the internal equipment such as watering apparatus in the facility, or environment-controlling instrument for regulating the temperature, concentration of nutrients, light intensity, etc., in the facility. On the other hand, in the above-mentioned open-air field cultivation, in many cases, there is required a large investment in the irrigation and watering facilities, etc., for the purpose of overcoming the influence of sudden changes in the natural environment.
In general, the use of chemical-type fertilizers in place of organic-type fertilizers has been considered as one of the important factors which have supported the development of modern agriculture. However, the ratio of the chemical-type fertilizer which is to be actually absorbed into a plant is considered to be usually below 30%. Accordingly, in recent years, there is a fear such that soil is deteriorated and the environment is polluted globally due to the use of the chemical-type fertilizer, and the natural resources to be used as the raw material for producing the chemical-type fertilizer are even exhausted.
In order to solve the above-mentioned problems, there has strongly been demanded a method of effectively applying a fertilizer to a plant, or an improvement in "carrier for supporting a plant" such as soil with which a fertilizer is effectively applied to a plant.
In recent years, in addition to the above-mentioned problem in the "carrier", the improvement in a "vessel" for growing a plant also becomes an important subject in the field of the plant growth.
Heretofore, the transfer of a plant is required to be conducted by long-term troublesome manual operations. Further, there are pointed out various problems such that the above-mentioned manual operations damage the plant, or they considerably impair the initial (or early-phase) growth of the plant after the transfer operation, etc. From such a viewpoint, it has strongly been desired to automate the transfer operation of a plant, to reduce the transferring damage (or rooting damage) at the time of the transfer operation, or to promote the initial growth of the plant after the transfer operation.
In addition, the conventional vessel for growing a plant has a problem such that it is liable to deteriorate the physical environment of the rhizosphere of the plant. In general, when the root of a plant is extended to the wall surface of a vessel, it is further extended downward along the wall surface, and when the root reaches the bottom surface of the vessel, it is extended so as to be formed into a coil-like shape along the bottom surface in many cases. It is considered that such a phenomenon of root extension is based on the property of the root of extending in the direction of the gravity, and the property thereof of extending due to the contact surface stimulus to be applied to the root. On the other hand, in the cultivation of a plant using a vessel, it has heretofore been investigated particularly intensively to find what kind of supporting carrier (such as soil) is optimum in view of the growth of the plant. Accordingly, the investigation of the physical environment of internal surface of the vessel has been insufficient.
In general, the physical environment of the interior of the carrier for retaining a plant is utterly different from that of the internal surface of the vessel. In the latter environment, i.e., in the neighborhood of the internal surface of the vessel, since the difference in the temperature (hot and cold) and humidity (dry and wet) is large, there is particularly liable to occur a problem such that the growing point of the root which contacts the internal wall surface, or an intermediate portion of the root which is extended along the internal wall surface, causes browning and fatal withering.
Heretofore, specific examples of the vessel or sheet to be used for the growth of a plant, include unglazed (earthenware) pots, plastic pots, vinyl pots, planters, trays for "plug-type" plantlet, trays for plantlet, paper pots, vinyl sheets, etc. In any of these vessels, when the material of the vessel (such as usual plastic material) is one which intercepts the circulation of water or outside air, the place in the neighborhood of the internal wall surface of the vessel is a place wherein water is liable to be accumulated, and a root decay phenomenon is liable to occur. On the other hand, when the material of the vessel is one which enables the circulation of outside air (such as unglazed earthenware and paper), on the contrary, the place in the neighborhood of the internal wall surface of the vessel is a place wherein water is liable to be insufficient, and the growth of the plant is liable to be markedly hindered.
In addition, the vessel to be used in the farm cultivation is generally one having an open-system portion in the upper and lower parts thereof. Accordingly, after a watering operation, the supplied water is promptly drained from the lower part of the vessel through the plant-supporting carrier, thereby to cause problems such as the necessity for excessive watering operations and the flowing-out of nutrients. Further, in the case of a vessel to be used in general homes, a "pan" for receiving water, etc., drained from the lower part of the vessel is required.
Further, when such an open-type vessel is used, since the amount of water immediately before the watering operation is liable to be sharply decreased suddenly, the concentration of nutrients is sharply increased, whereby the plant can be adversely affected. On the other hand, when the amount of water to be used in the watering operation is increased, the frequency of the watering operation is increased, or the conventional vessel and soil having a high water-retaining property are selected for the purpose of improving such a situation, the accumulation of excessive water lasting immediately after the watering inhibits the supply of oxygen to the root, and microorganisms adversely affecting the plant are propagated, whereby a root decay phenomenon is liable to occur. On the contrary, when the plant-growing system is dried for the purpose of promoting the supply of oxygen, or of suppressing the propagation of microorganisms adversely affecting the plant, the above-mentioned problems of a decrease in the water content and the concentration of nutrients become severer, and therefore a fertilizer must be supplied to the plant in a smaller amount than that of the fertilizer required by the plant.
As described above, the present plant cultivation using a plant-growing vessel is caught in a vicious circle or falls into a self-contradiction, and therefore such a condition makes it difficult to cause a plant to exhibit its growing power to the maximum.
Further, the upper portion of the above-mentioned open-type vessel is in a completely open state, and therefore the upper portion of a plant-supporting carrier is liable to be dried than the lower portion thereof so that aqueous or moisture condition in the vessel becomes ununiform and such a condition is liable to adversely affect the plant.
On the other hand, it is not usual to use a vessel having a closed-type lower portion. The reason for is that when the conventional vessel is simply used as one having a closed-type lower portion in combination with the conventional soil, the residence or retention of water becomes marked, and such a phenomenon is extremely liable to cause the above-mentioned insufficient oxygen supply, and root decay phenomenon due to the propagation of pathogens.
As described above, in the conventional plant-growing vessel or sheet, the environment of the rhizosphere is not suitable for the growth of a plant (inclusive of the germination of a seed and the growth thereof after the germination). Accordingly, when such a conventional vessel or sheet is used, there is required a complicated combination or regulation of the amount of water for the watering, the frequency of the watering, the concentration of solutions such as fertilizer, etc., and therefore the strict control of these factors has required great costs.
In general, with respect to various kinds of plant, the internal volume of the most suitable growing vessel has been known empirically depending on the kind, size thereof, etc. It is said that when the root of a plant is grown in soil so as to reach the wall of the growing vessel, the resultant mechanical contact stimulus promotes the origination of a new root. From such a viewpoint, the origination of root of the plant becomes better as the volume of the vessel becomes smaller. On the other hand, the internal volume of the vessel closely relates to the amount of water and nutrients stored therein which are to be supplied to the plant, and therefore, in general, the volume of the vessel of not lower than a certain level is required for the growth of the plant.
The circumstances of general cultivation farmhouses are those as described above. In recent years, however, in general homes, a many kinds of plant-growing vessels have been used in so-called "private (vegetable) gardens". In general homes, since they do not have skilled experience or technique, unlike in the cultivation farmhouses, it is further difficult to suitably grow the plant, as compared with in the case of the cultivation farmhouses.
An object of the present invention is to provide a plant-cultivating support or a soil-modifying (or reforming) agent, which can solve the above-mentioned problems encountered in the prior art.
Another object of the present invention is to provide a plant-cultivating support or a soil modifying agent, which can suitably control the amount of water, the amount of nutrients, or the amount of a plant growth-regulating substance to be supplied to a plant so as to meet the demand of the plant, in accordance with a change in the external environmental factors such as temperature.
A further object of the present invention is to provide a plant-cultivating support or a soil modifying agent, which can enhance the productivity by saving labor to be required in open-airfield cultivation and cultivation in facilities, and by reducing the facility costs.
A further object of the present invention is to provide a method of effectively growing a plant while using a plant-cultivating support or a soil-modifying agent.
A further object of the present invention is to provide a vessel or sheet for cultivating a plant, which can solve the above-mentioned problems of the conventional plant-cultivating vessel.
A further object of the present invention is to provide a plant-cultivating vessel or sheet, which can automate the transferring operation for a plant, and further can reduce the transferring damage to the plant at the time of the transferring operation.
A further object of the present invention is to provide a plant-cultivating vessel or sheet, which can suitably control the environment in the rhizosphere of the plant, while strict control of water, etc., is not necessarily required.
A further object of the present invention is to provide a plant-cultivating vessel or sheet, which has an ability to store water and/or nutrients which are necessary for the germination or growth of a plant.
A further object of the present invention is to provide a method of cultivating a plant, which has solved the above-mentioned problems encountered in the prior art.
A still further object of the present invention is to provide a method of cultivating a plant by using a support (or planting material) which can be used continuously from the culturing of the plant to the cultivation thereof.